So, in plain English, too much caffeine makes us sleepy? Less able to concentrate?

You'd think so, but it's fairly easy to demonstrate the opposite (in terms of sleepiness, anyways). I would postulate that stimulant-induced elevated heart rate would counteract the vasoconstriction in the brain, but that's just me spitballing.

No, the plain English is much more serious: caffeine makes head trauma significantly more dangerous.

When your brain takes an impact, chemicals spill out from inside your neurons to the fluid surrounding them. The fancy word for this is "depolarization", and in this case, it's bad news.

Pumping the chemicals back into the neurons where they belong takes extra energy beyond what your brain normally needs to operate.

To generate energy, your brain normally converts glucose into water and carbon dioxide. To do this, it needs oxygen, which comes from the blood pumped to the brain. The wider the blood vessels are, the more blood flows through them and the more oxygen the brain can get from them.

When you get hit in the head, the blood vessels in your brain get narrower. Narrower vessels carry less blood, less blood holds less oxygen, and less oxygen means less energy to fix up your brain and keep your neurons alive.

Caffeine makes the blood vessels even narrower. Even less blood, even less oxygen, and even less energy to keep your brain cells from dying.

Interesting paper. There's some criticism for his findings, but I wouldn't be shocked to learn that sorghum figured into their diet in some form. In particular, given the energy inefficiency involved with processing grass, I'm led to wonder if they weren't open-air fermenting it into a kind of lambic (primates will go a long way for a buzz).

BTW: the ~12k YBP number people keep using for the introduction of grain to the diet has already been trashed by milled grass seeds found at an Israeli site dating ~23k YBP.

“Most people do not do, but take refuge in theory and talk, thinking that they will become good in this way” -- Aristotle, Nicomachean Ethics, II.4

So I saw ATP tablets on a nutrition-supply site today. They're running $0.60/tablet for 125mg of the stuff. Seems... a bit pricey, especially since this is the only literature I could find on the subject:

Twenty-seven healthy males successfully completed the trial, after randomly receiving in a double-blind manner an oral dose of low dose (150 mg) or high dose (225 mg) ATP, or matched placebo. To improve absorption characteristics, the ATP was enterically coated. Total blood ATP (whole blood and plasma ATP) concentrations, two Wingate anaerobic power tests (30 s), and muscular strength (1RM and three sets of repetitions to fatigue at 70% of 1RM) were measured under three conditions: (i) baseline; (ii) acutely (7d later, no prior supplementation and 75 min after ATP ingestion); and (iii) after 14 d of daily ingestion (post). RESULTS: Statistical analyses showed no significant between or within group treatment effects for whole blood ATP or plasma ATP concentrations for any treatment condition. We also did not observe any treatment effects for any Wingate testing parameter including peak PO, total work, average PO for 30 s, or post-Wingate lactate accumulation. Overall, we observed no significant between group treatment effects for any muscular strength parameter. We did observe several within group differences for the group ingesting the high ATP dosage including 1RM (6.6%; P < 0.04) and repetitions to fatigue during set 1 of posttesting (18.5%; P < 0.007) and total lifting volume at post (22%; P < 0.003). CONCLUSIONS: We conclude that enterically coated oral ATP supplementation may provide small ergogenic effects on muscular strength under some treatment conditions.

First time actually lifting weights in about eleven months. Weights were insubstantial, as one might expect after a year of detraining, but it's a blessing to be able to lift, and I'm going to try to remember that going forward.